Cooperative and substitution effects in enhancing strengths of halogen bonds in FCl⋯CNX complexes

Enhancement of the Tetrel Bond by the Effects of Substituents, Cooperativity, and Electric Field: Transition from Noncovalent to Covalent Bond

The T⋅⋅⋅N tetrel bond (TB) formed between TX₃ OH (T=C, Si, Ge; X=H, F) and the Lewis base N≡CM (M=H, Li, Na) is studied by ab initio calculations at the MP2/aug-cc-pVTZ level. Complexes involving TH₃ OH contain a conventional TB with interaction energy less than 10 kcal/mol. This bond is substantially strengthened, approaching 35 kcal/mol and covalent character, when fluorosubstituted TF₃ OH is combined with NCLi or NCNa. Along with this enhanced binding comes a near equalization of the TB T⋅⋅⋅N and the internal T-O bond lengths, and the associated structure acquires a trigonal bipyramidal shape, despite a high internal deformation energy. This structural transformation becomes more complete, and the TB is further strengthened upon adding an electron acceptor BeCl₂ to the Lewis acid and a base to the NCM unit. This same TB strengthening can be accomplished also by imposition of an external electric field.

Comparison between Chlorine-Shared and π-Halogen Bonds Involving Substituted Phosphabenzene and ClF Molecules

Ab initio MP2/aug-cc-pVTZ calculations have been carried out in order to study the nature of P···Cl halogen bonding interaction between a phosphorus atom in an aromatic ring in para-substituted phosphabenzene (PPBZ) and ClF molecule. The interaction of PPBZ with ClF results in two different types of complexes: (i) complex formation through the chlorine-shared halogen bond (T1-X-PPBZ·ClF) and (ii) complex formation via halogen-π interaction (T2-X-PPBZ·ClF). T1-X-PPBZ·ClF complexes are found to be more stable than the T2-X-PPBZ·ClF complexes. This work also presents a general criterion to distinguish a chlorine-shared halogen bond from a traditional halogen bond and sheds light on the formation of the chlorine-shared halogen bond. The binding energy of T1-X-PPBZ·ClF complexes correlates well with the negative electrostatic potential of the P atom and PA value of the substituted PPBZ. The properties of both T1-X-PPBZ·ClF and T2-X-PPBZ·ClF complexes are analyzed using atom-in-molecule, natural bond orbital, and symmetry-adapted perturbation theory calculations. The variation of the Cl-F bond distances and the redshifts of the ν(ClF) vibration resulting from the interaction with PPBZs are discussed.

Halogen Bonding Involving CO and CS with Carbon as the Electron Donor

MP2/aug'-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH₂. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC:ClNC and SC:ClCl surfaces. SC:ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC:ClY complexes. The largest binding energies are found for the ion-pair SC-Cl⁺:-Y complexes. The transition structures which connect the complex and the ion pair on SC:ClNC and SC:ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC:ClY and SC:ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY. This secondary interaction assumes increased importance in the SC:ClNH₂ complex, and is a factor leading to its unusual structure. C-O and C-S stretching frequencies and 13C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC:ClY complexes and SC-Cl⁺:-Y ion pairs. Spin-spin coupling constants 1xJ(C-Cl) for OC:ClY complexes increase with decreasing distance. As a function of the C-Cl distance, 1xJ(C-Cl) and ¹J(C-Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.

Modeling cooperative effects in halogen-bonded infinite linear chains

Non-additivity in noncovalent interactions is an important aspect of complex systems that can lead to stronger (cooperative) interactions when three or more molecular units influence each other. The halogen bond (XB) is a highly-directional noncovalent interaction that has been found to be cooperative. Here the strength and nature of cooperativity arising in X-bonded infinite linear chains of cyanogen halides and 4-halopyridines are investigated by means of density functional theory calculations. It is found that cyanogen halide chains are highly cooperative (up to 77%), whereas pyridines are only slightly cooperative (below 21%). It is demonstrated that XB and its non-additivity can be modeled as the sum of a local term, which depends on first nearest-neighbors only, and long-range effective dipole-dipole attractions. It is shown that the local term in cyanogen halides primarily accounts for repulsive short-range screened Coulomb interactions, whereas in 4-halopyridines such a term also includes attractive contributions, which are particularly sizeable in some elongated XB conformations. This outcome reveals differences in the nature of the XBs formed in these molecular systems. Nevertheless, it is shown that both systems behave as effective point dipoles regarding cooperative effects, at any point of the XB dissociation path. As such, these results are useful contributions for the understanding and modeling of non-additive effects of noncovalent interactions.

Ion-Pair Halogen Bonds in 2-Halo-Functionalized Imidazolium Chloride Receptors: Substituent and Solvent Effects

The interaction of 2-halo-functionalized imidazolium derivatives (n-X⁺ ; X=Cl, Br, I) with a chloride anion through ion-pair halogen bonds (n-X⋅Cl) was studied by means of DFT and ab initio calculations. A method benchmark was performed on 2-bromo-1H-imidazol-3-ium in association with chloride (1-Br⋅Cl); MP2 yielded the best results when compared with CCSD(T) calculations. The interaction energies (ΔE) in the gas phase are high and, although the electrostatic interaction is strong owing to the ion-pair nature of the system, large X⋅⋅⋅Cl^- Wiberg bond orders and contributions from charge transfer (nCl- →σ*C-X) are obtained. These values drop considerably in chloroform and water; this shows that solvent plays a role in modulating the interaction and that gas-phase calculations are particularly unrealistic for experimental applications. The introduction of electron-withdrawing groups in the 4,5-positions of the imidazolium (e.g., -NO₂ , -F) increases the halogen-bond strength in both the gas phase and solvent, including water. The effect of the substituents on the 1,3-positions (N-H groups) also depends on the solvent. The variation of ΔE can be predicted through a two-parameter linear regression that optimizes the weights of charge-transfer and electrostatic interactions, which are different in vacuum and in solvent (chloroform and water). These results could be used in the rational design of efficient chloride receptors based on halogen bonds that work in solution, in particular, in an aqueous environment.

Cooperative and substitution effects in enhancing the strength of fluorine bonds by anion−π interactions

In this work, the cooperative effects between anion−π and fluorine bond interactions are studied by ab initio calculations at the MP2/6-311++G** level. Cooperative effects are observed in complexes in which anion−π and fluorine bond interactions coexist. For each complex, the shortening of the binding distance in the fluorine bond is more prominent than that in the anion−π bond. Favorable cooperativity energies are found with values that range between –0.51 and –0.76 kcal/mol. The atoms in molecules and molecular electrostatic potential analyses are carried out for these complexes to understand the nature of anion−π and fluorine bond interactions and the origin of the cooperativity.

Using beryllium bonds to change halogen bonds from traditional to chlorine-shared to ion-pair bonds

Dramatic synergistic cooperative effects between Be⋯F beryllium bonds and Cl⋯N halogen bonds in XYBe:FCl:N-base ternary complexes lead to changes in the halogen-bond type from traditional to chlorine-shared to ion-pair bonds.

Traditional and ion-pair halogen-bonded complexes between chlorine and bromine derivatives and a nitrogen-heterocyclic carbene

A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug'-cc-pVDZ level of theory. Two types of A-X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C-X distances and small binding energies (8-54 kJ·mol(-1)). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C-X distances and large binding energies (148-200 kJ·mol(-1)), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A(-) and (+)[X-CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C-X bond is also identified as a significant stabilizing interaction in type-II complexes.

Theoretical investigation on the interplay of hydrogen bond and halogen bond in HX⋯(BrCl)n (X = F, Cl, Br and n = 1, 2) complexes

Quantum chemical calculations at the MP2 level with the aug-cc-pVTZ basis set were used to investigate the HX ⋯( BrCl )n ( X = F , Cl , Br and n = 1, 2) complexes. They are connected via hydrogen bond or halogen bond in dimers and together in trimers. Molecular geometries, interaction energies, cooperative energies and atomic charge of dyads and triads have been studied at the Mp2/aug-cc-pVTZ computational level. All studied trimers show cooperativity with the simultaneous presence of a hydrogen and one or two halogen bond. The molecular electrostatic potential has been employed to explore the formation mechanisms of these molecular complexes. The AIM analysis has been performed at the Mp2/aug-cc-pVTZ level to examine the topological characteristics, confirming the coexistence of hydrogen bonds and one or two halogen bond for each complex.

COOPERATIVITY BETWEEN HYDROGEN BOND AND HALOGEN BOND IN HBr⋯BrX⋯HBr (X = F, Cl, Br)

An ab initio study has been carried out to investigate the structures, binding energies and atomic charge of the binary complexes HBr⋯BrX and ternary complexes HBr⋯BrX⋯HBr (X = F, Cl, Br) at the MP2/aug-cc-pvdz theoretical level. Four types of interactions are observed among these systems: hydrogen bond, halogen bond, unusual hydrogen bond and unusual halogen bond. Two types of stable structures existed in the trimers: acyclic and monocyclic. The stabilities of the complexes may be decided by three factors: the strength of the halogen bond, the nature of halogen or hydrogen bond of intermolecular interaction and the type of the trimer's structure. The absolute value of the cooperativity contribution in these triads is in range of 0.77 ∼ 21.59%, which shows that the cooperative energy is of importance in them. The nature of the intermolecular interactions has been also investigated through the electrostatic potential and the theory of atoms in molecules (AIM). The values of ∇2ρ at BCPs are positive in all complexes, indicating that the interactions are closed-shell. The ratio |Vc|∕Gc of all complexes ranges from 0.782 to 0.935 (satisfy the condition |Vc|∕Gc < 1) along with Hc > 0, revealing that these complexes are predominantly electrostatic in character.